Titanium is a light metal having a high specific strength and exhibiting excellent corrosion resistance. Titanium is widely used in various fields including airplane, medical and automobile industries. An amount of titanium in use has been increasing. Titanium is a plentiful resource and the fourth most abundant element in the earth's crust after aluminum, iron, and magnesium in metal elements. Although titanium is a plentiful resource, titanium has been at least an order of magnitude more expensive than steel materials and is up against short supply.
Currently, titanium metal has been mainly produced by a Kroll Process. In the Kroll Process, titanium ore, the main component of which is titanium dioxide (TiO2), is reacted with a chlorine gas and coke (C) to provide titanium tetrachloride (TiCl4). Subsequently, highly-purified titanium tetrachloride is produced through distillation and separation. Titanium metal is produced by thermal reduction of the purified titanium tetrachloride and magnesium (Mg). In a thermal reduction step of the Kroll Process, a reduction reaction vessel made of stainless steel is filled with a magnesium melt at the temperature of not lower than 800° C. Titanium tetrachloride in a liquid phase is dropped into the vessel from the above and reacts with the magnesium melt in the vessel to produce titanium. The produced titanium sinks in the magnesium melt and thus the titanium is produced in a sponge form. By-product titanium tetrachloride and unreacted magnesium in the liquid phase are mixed with the titanium in the sponge form. Upon completion of the reaction, the reaction mixture is subjected to a vacuum separation process at a high temperature of not lower than 1000° C. to obtain a sponge cake of porous titanium. The sponge cake is crushed to produce sponge titanium.
By the Kroll Process, a titanium material can be produced for practical use. However, a long production time is required since the thermal reduction process and the separation process are preformed separately. Also, the processes are performed batch-wise and have low production efficiency. Accordingly, various techniques have been suggested to overcome the problems of the Kroll Process.
For example, Patent Literature 1 (JP-B-33-3004) discloses a method for collecting titanium including supplying a titanium tetrachloride gas and magnesium in a reaction vessel to cause a gas-phase reaction under a temperature range of 800 to 1100° C. and a vacuum of 10−4 mmHg (1.3×10−2 Pa) in the vessel and depositing titanium on a net-like collection material disposed in the vessel.
Patent Literature 2 (U.S. Pat. No. 2,997,385) discloses a method for producing metal including introducing halide vapor as a metal element and alkali metal or alkaline earth metal vapor as a reducing agent into a reaction vessel to cause a gas-phase reaction in the vessel in an evacuated state under a temperature range of 750 to 1200° C. and a pressure of 0.01 to 300 mmHg (1.3 Pa to 40 kPa).
Example II disclosed in Patent Literature 2 discloses that titanium was produced by TiCl4 gas and Mg gas. More specifically, the reaction was caused under a reaction temperature of approximately 850° C. and a pressure of 10 to 200 microns (1.3 to 26.7 Pa).
Non Patent Literature 1 (D. A. Hansen and S. J. Gerdemann, JOM, 1998, No. 11, page 56) discloses a method for producing titanium ultrafine powders through a gas-phase reaction. According to the method, titanium tetrachloride gas and magnesium gas are introduced into a reaction vessel and reacted at a temperature of not lower than 850° C., and produced titanium ultrafine powders and concomitantly produced MgCl2 powders are separated in a cyclone provided on a lower portion. Then, magnesium and MgCl2 are separated from the obtained titanium ultrafine powders through vacuum distillation or filtration.